1. Field of the Invention
This invention relates to devices for measuring the rotation and angular position of a shaft and more particularly to devices which use optical means for measuring the rate of rotation.
2. Description of the Related Art
In the past many methods were used to detect the rate of rotation of a shaft. Mechanical means have been used extensively. In more recent years electronic means such as a magnetic implant in a shaft and a magnetic field detector adjacent the shaft to detect the passing magnetic field have been used. These electronic means for measuring the speed of the shaft are subject to electromagnetic interference which reduces the reliability of the device. Optical means for measuring the speed of a rotor are not subject to electromagnetic interference. In the past optical means for measuring the speed of a rotor have included a means to polarize light as a function of shaft rotation position and a diffraction grating where light of a specific frequency reflected from the grating is detected indicating an angular position in the rotation of the wheel.
Many types of rotary measurement devices experience errors in the readings due to temperature fluctuations, light amplitude fluctuations and slippage or creep of the devices measurement elements.